EP 2450673 B1 describes an optical position-measuring device that is based on scanning a material measure with light. To this end, light from a light source in a scanning head is passed through a transparent scanning reticle onto a scale, reflected back therefrom to the scanning head and detected in a sensor unit. By analyzing the sensor signals, highly accurate position values are determined which are indicative of the displacement to be measured between the scanning head and the material measure. For certain functions, it is even necessary that the light impinge on the material measure several times. Therefore, the scanning reticle is provided on both its top side and on its underside with different functional surfaces that act as a reflector or mirror or as grating structures (e.g., in the form of diffraction gratings or diffractive lenses) as well as surfaces that are provided with an antireflection coating to absorb disturbing scattered light inside and outside of the scanning reticle.
Mirrors may be produced, for example, by vapor deposition of metals on the transparent substrate of a scanning reticle. Also possible are dielectric mirrors which have a reflective effect obtained by a combination of layers having refractive indices and thicknesses that are carefully selected and matched to the wavelength of the light used.
Optical gratings may take the form of phase gratings which are composed of two alternately arranged reflective layers, one of which having an optically retarding spacer layer that causes a 180 degree phase shift toward the zeroth diffraction order. In contrast, amplitude gratings are based on alternating bright and dark regions, such as, for example, in a structured metallic layer. Examples of optical gratings and their application in position-measuring devices are disclosed in EP 0742455 A1 (phase grating) and DE 10236788 A1 (amplitude grating).
On the other hand, antireflection coatings are composed of layers that have different optical densities and absorb scattered light.
The manufacture of such layers or layer stacks having a wide variety of different functions is complex technologically and from a process point of view. Each layer must meet the highest quality standards to ultimately enable as accurate a position measurement as possible.